With an upsurge in recent years of the environmental concern and the pursuit of better fuel economy, hybrid electric vehicles have been put in the market. Such vehicles are driven not only by engines but also by electric motors, hence improving energy efficiencies and reduce their fuel costs. Those hybrid electric vehicles require an electric power storage unit capable of storing high power for driving the motors. The electric power storage unit has a function for not only driving the motors but also storing energy regenerated during braking. This function reduces the fuel costs by the effective use of braking energies.
The electric power storage unit repetitively charges and discharges electricity in a short period while the vehicle is driven. This operation produces heat due to an internal resistance of the electric power storage element, such as a secondary battery or a capacitor, for storing electric power. Upon remaining, the heat may have the electric power storage element deteriorate and shorten its life time, thus decreasing reliability.
An electric power storage unit cooling an electric power storage element is disclosed in Patent Document 1. FIG. 24 is an exploded perspective view of the electric power storage unit.
In FIG. 24, electric cell 501 composed of a secondary battery is used as the electric power storage element for storing electric power. Plural electric cells 501 (six cells shown in FIG. 24) are connected to form battery module 503. A number of battery modules 503 are stacked in multiple rows and multiple stages to provide a battery module unit 505 to provide predetermined power. Battery module unit 505 is fixed to case body 507. Battery modules 503 are electrically connected to one another with bus bars 511 and screws 513 fixed to screw holes in terminals 509 provided on respective ends of battery modules 503. In addition, case body 507 is provided with fan 515 for cooling battery module unit 505. Cover 517 is fixed to case body 507, thus providing the electric power storage unit.
The above electric power storage unit includes fan 515 operating while the vehicle is used for cooling battery module unit 505, hence having reliability.
The above electric power storage unit is indeed capable of cooling battery module unit 505 and has high reliability by operating fan 515 when the vehicle is used. However, even when fan 515 delivers air to battery module unit 505, the air does not necessarily cools each electric cell 501 as well. In other words, while electric cells 501 near fan 515 are indeed cooled effectively, the cooling air is not delivered sufficiently to electric cells 501 located far from fan 515. Electric cells 501 located far from fan 515 receives air warmed by electric cells 501 near to fan 515, hence reducing cooling efficiency. As a result, variations in the degrading of the cells, i.e., in their life time since effects of the cooling are different among electric cells 501. In view of the variations of degradation of the individual electric cells 501, the above-discussed structure is therefore not sufficient to ensure the reliability although it has achieved a certain degree of improvement in the reliability compared with the conventional structures not having the cooling function.
Patent Document 1: Japanese Patent No. 3681051